1. Field of the Invention
The present invention relates to a fast response high purity membrane nitrogen generator.
2. Description of the Background
Membrane processes are used increasingly to remove a desired single gas from a gaseous feed mixture. For example, such processes may be used to remove water vapor from a moist air feed to produce a dry air product. As water vapor is more rapidly permeated through the membranes than other gases, the non-permeate product has a lower concentration of water vapor than does the feed.
Membrane processes may also be used to produce inert gases from air, wherein the oxygen content in the feed air stream to the membrane is decreased by permeation from a high pressure side to a low pressure side. Generally, such membrane processes are advantageous due to their simplicity and ease of operation.
However, membrane processes used for the production of inert gases from atmospheric air are disadvantageous as they are generally non-responsive to variation in customer demand. For example, membrane nitrogen generators are presently used to feed customer lines where demand may diminish to zero several times per day.
When demand reaches zero and the feed air compressor stops, remaining high pressure feed air continues to flow across the membrane wall until pressures equalize. If the unit remains stopped for a considerable period of time, impurities in ambient air will eventually permeate back to the high pressure side of the membrane. When the unit is restarted in response to customer demand, back-permeated gas containing impurities will be first compressed by incoming feed air and then fed to the production line.
At present, production gas is vented to the atmosphere when the unit is restarted in order to avoid feeding polluted gas to the customer line. The venting either continues during a preset time delay so that production quality can be recovered or until the quality control meter indicates that the gas quality is sufficiently good. However, venting can lead to waste of costly gas and energy, particularly when low oxygen content and/or low dew point inert gas is produced.
Recently, a process was disclosed in EP 0,426,642 A2 for using a membrane gas separator in generally separating a portion of a first gas from an intermittently supplied gaseous feed containing a mixture of gases. In accordance with this process, the gaseous feed is fed intermittently under pressure through a membrane gas separator through one or more membranes through which the first gas permeates preferentially in comparison to other gases in the mixture to produce a non-permeate gas product which is discharged from a non-permeate gas side of the separator, wherein the concentration of the first gas is lower than in the feed mixture. Then, a gaseous purge stream is supplied to the separator when the feed mixture is not being conducted through the separator, with the concentration of the first gas being lower in the gaseous purge stream than in the feed mixture, whereby residual amounts of the first gas contained in the membrane of the separator are purged.
Notably, however, EP 0,426,642 A2 relates only to gas production in general or to the specific production of dehydrated air, and does not pertain to the specific production of nitrogen from atmospheric air or to the specific separation of nitrogen from oxygen.
Unfortunately, the variation in customer demand is particularly troublesome for the production of nitrogen. For example, if a "significant" period of generator use is considered, defined as the combined cycle duration of one period of production plus one period of generator down-time, generally the generator down-time may represent from about 1% to 50% of the overall total cycle.
However, in order for a backflush across the membrane to be acceptable to the customer on a cost basis as a means of preventing retrodiffusion of permeate gas across the membrane, the backflush must be a negligible volume of the gas produced by the generator, which means less than 10% of the overall volume of gas produced, and preferably less than 1%.
Further, while EP 0,426,642 A2 describes supplying a purge stream to the separator at times when the feed is not being supplied to the separator, the purpose of this purge stream is to remove residual water vapor to prevent it from being entrained in the non-permeate product when the gaseous feed is restarted, in order to improve removal efficiency.
Moreover, this purge stream is supplied to both sides of the separator with no distinction being made between permeate and non-permeate sides of the membrane.
Additionally, EP 0,426,642 A2 describes the use of either a primary pressure vessel alone or that in combination with a secondary pressure vessel. Generally, the primary pressure vessel is connected directly downstream of the non-permeate line via a check valve, whereas the secondary pressure vessel is connected to the non-permeate line on a side stream conduit via a flow orifice. Regardless of whether one or two pressure vessels are used, however, the function of the check valve is to allow relatively dry stored non-permeate product to bleed there through to the separator to purge residual amounts of water vapor contained in the membrane. Thus, when the compressor is cycled back on, the residual water vapor will not be entrained in the non-permeate product.
Additionally, EP 0,426,642 A2 neither describes nor suggests means for specifically feeding product gas to a high pressure side of a membrane in order to maintain a positive pressure on the feed inside thereof, particularly with a negligible loss of product gas, thereby avoiding back permeation or retrodiffusion of gas to the feed side of the membrane.
Thus, at present, gas generators in use are either for the production of gas, in general, or for dehydrated air, in particular, and are not generally constructed to address the specific problem of variation in customer demand in the production of nitrogen. Moreover, conventional gas generators often lose significant amounts of product gas by venting the same upon being restarted. Furthermore, such generators are generally incapable of being quickly restarted.
Therefore, a need exists for a membrane nitrogen generator which is capable of being instantly restarted after being down without venting of initial production, or without initial reduction of production volume.